$-\dfrac{5}{8} + \dfrac{8}{5} = {?}$
$ = - {\dfrac{5 \times 5}{8 \times 5}} + {\dfrac{8 \times 8}{5 \times 8}} $ $ = - {\dfrac{25}{40}} + {\dfrac{64}{40}} $ $ = - \dfrac{{25} + {64}}{40} $ $ = \dfrac{39}{40}$